Drive circuit for insulated gate switching element

ABSTRACT

Embodiments of the invention provide a drive circuit including: a constant current source that generates a constant current; a switching circuit that connects a gate of the insulated gate switching element to a power supply potential side via the constant current source when turning the insulated gate switching element ON and connects the gate of the insulated gate switching element to a reference potential side via a discharge circuit when turning the insulated gate switching element OFF; a gate voltage detection circuit that detects a gate voltage of the insulated gate switching element; and a current mode selection circuit that switches a mode of the constant current source from a normal current mode to a low current consumption mode when detecting, based on the gate voltage detected by the gate voltage detection circuit, that the insulated gate switching element is turned ON.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/JP2012/002425, filed on Apr. 6, 2012, which is based on and claimspriority to Japanese Patent Application No. JP 2011-106182, filed on May11, 2011. The disclosure of the Japanese priority application and thePCT application in their entirety, including the drawings, claims, andthe specification thereof, are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

Embodiments of the invention relate to drive circuits for drivinginsulated gate switching elements such as an IGBT (Insulated GateBipolar Transistor) or MOSFET (Metal Oxide Semiconductor Field EffectTransistor).

2. Related Art

A semiconductor device that has packaged therein an insulated gateswitching element, such as an IGBT, and a drive circuit for driving theswitching element is called “IPM (Intelligent Power Module)”. A powerswitching element such as an IGBT is mounted on the IPM for driving amotor or the like. Application of excess current to this power switchingelement can immensely damage an electronic device equipped with the IPM.For this reason, the semiconductor device is provided with a selfprotection function that constantly monitors the current flowing to thepower switching element and safely stops the control by discontinuingthe supply of gate signals when an excess current beyond a predeterminedcurrent value flows to the power switching element.

Such a conventional IPM adopts an IGBT drive system that connects aP-channel MOSFET 51 and an N-channel MOSFET 52 in series, as shown inFIG. 4. The source of the P-channel MOSFET 51 is connected to a powersupply voltage Vcc, while the source of the N-channel MOSFET 52 isconnected to a ground potential. The drains of the P-channel MOSFTET 51and the N-channel MOSFET 52 are connected to the gate of an IGBT 53, anda drive signal is input to the gate of the P-channel MOSFET 51 and thegate of the N-channel MOSFET 52.

When turning the IGBT 53 ON, the level of the drive signal is lowered sothat the P-channel MOSFET 51 is turned ON and the N-channel MOSFET 52 isturned OFF. Consequently, the power supply voltage Vcc is applied to thegate of the IGBT 53 via the P-channel MOSFET 51.

When turning the IGBT 53 OFF, on the other hand, the level of the drivesignal is increased so that the P-channel MOSFET 51 is turned OFF andthe N-channel MOSFET 52 is turned ON. Consequently, the ground potentialis applied to the gate of the IGBT 53 via the N-channel MOSFET 52.

In this configuration, ON-resistances of the P-channel MOSFET 51 and theN-channel MOSFET 52 are used for driving the IGBT 53 so as to turn theIGBT 53 ON and OFF.

In this system where the IGBT 53 is driven by the ON-resistances of theP-channel MOSFET 51 and the N-channel MOSFET 52, the ON-resistances ofthe P-channel MOSFET 51 and the N-channel MOSFET 52 increase at atemperature higher than a room temperature. Therefore, when thetemperature is higher than the room temperature, the charge rate at thegate of the IGBT 53 slows down and a steep voltage change (voltagebetween the collector and the emitter of an IGBT 43) is prevented,alleviating the occurrence of noise related to the voltage change.Nevertheless, the problem is the increase in losses due to an increasein time required to turn the IGBT 53 ON. However, when the design of thesemiconductor device is optimized in order to minimize the losses inhigh temperatures, the charge rate at the gate of the IGBT 53 becomesextremely low at the room temperature, causing a steep voltage changeand consequently increasing noise.

In order to solve these problems, Patent Document 1 proposes a drivecircuit for an insulated gate device.

In Japanese Patent Application Publication No. 2008-103895 (alsoreferred to herein as “Patent Document 1”), the drive circuit for anIGBT is provided with a constant current source generating a constantcurrent and configured by a current mirror circuit, a switching circuitthat connects a gate of the IGBT to a power supply potential side viathe constant current source when turning the IGBT ON and disconnects thegate of the IGBT from the power supply potential side via the constantcurrent source when turning the IGBT OFF, and a discharge circuit thatturns the IGBT OFF. When the level of a drive signal becomes low, theswitching circuit connects the gate of the IGBT to the power supplypotential side via the constant current source, to turn the IGBT ON.When, on the other hand, the level of the drive signal becomes high, theswitching circuit terminates the connection made between the gate of theIGBT and the power supply potential side via the constant currentsource, and the discharge circuit connects the gate of the IGBT to aground potential to turn the IGBT OFF.

Japanese Patent Application Publication No. 2009-11049 (also referred toherein as “Patent Document 2”) proposes a gate drive circuit that has aconstant-current-pulse gate drive circuit that creates a gate signal fora voltage drive switching device such as an IGBT or FET as aconstant-current output, a constant-voltage-pulse gate drive circuitthat creates the gate signal as a constant voltage output, and a gatedrive circuit that has a decision/switch circuit that switches betweenan operation of the constant-current-pulse gate drive circuit and anoperation of the constant-voltage-pulse gate drive circuit.

Japanese Patent Application Publication No. 2005-260752 (also referredto herein as “Patent Document 3”) proposes a technology in which acurrent on the input side of a constant current source configured by acurrent mirror circuit is adjusted by bypassing one of theserially-connected resistors by means of a switch element, to change anoutput current of the constant current source.

Japanese Patent Application Publication No. 2000-40849 (also referred toherein as “Patent Document 4”) proposes a technology in which, as withPatent Document 3, a collector of one of PNP transistors configuring acurrent mirror circuit is connected to a terminal of a ground potentialvia a variable resistance resistor circuit, and a resistance of thevariable resistance resistor circuit is selected using a resistanceselecting part, to change an output current of the constant currentcircuit.

International Patent Publication No. WO 2008/155917 (also referred toherein as “Patent Document 5”) proposes a switching element drivecircuit that drives a switching element such as an IGBT or MOSFET forswitching large power. This switching element drive circuit uses a drivesignal output circuit for driving the switching element, to, first,output an increased voltage V2 to a gate of the switching element when aPWM pulse that is input from a PWM pulse output circuit is at a highlevel, and to, subsequently, output a predetermined voltage V1 lowerthan the increased voltage V2 to the gate of the switching element whena gate voltage Vgs at the switching element rises to a predeterminedvoltage. This configuration can prevent switching losses of theswitching element.

The demand for low current consumption in the IPM has been increasing,and the amount of current consumed by the IPM has a high proportion ofthe amount of current consumed by a drive circuit for driving an IGBTand a control circuit for protecting the IGBT.

In the conventional example described in Patent Document 1 above, theinsulated gate device can be turned ON via the constant current source,and the temperature dependence of the charge rate of the gate of theinsulated gate device can be reduced. Therefore, when turning theinsulated gate device ON, the noise caused in a high temperature periodand room temperature period can be minimized. Although being able tominimize the noise and losses, the conventional example described inPatent Document 1 has an unsolved problem of not being able to reducethe amount of current consumed by the drive circuit.

In the conventional example described in Patent Document 2, theinsulated gate device can be switched ON in a constant time period bydriving the insulated gate device at a constant current until the gatevoltage exceeds a predetermined voltage. In addition, by switching themode thereafter to a constant voltage drive mode, the device can bedriven without undermining the credibility of the gate oxide film of thedevice. However, Patent Document 2 has an unsolved problem of not beingable to reduce the amount of current consumed by the drive circuit.

Moreover, the conventional examples of Patent Documents 3 and 4 disclosethat the current values of the constant current circuits are changed bychanging the resistances, but do not at all describe drive circuits ofthe insulated gate switching elements.

In addition, according to the conventional example described in PatentDocument 5, when turning ON the switching element such as an IGBT orMOSFET for switching large power, an increased voltage, higher than apredetermined voltage applied constantly the gate, is applied only foran initial period until the gate voltage of the switching elementreaches the predetermined voltage. As a result, the switching operationcan be performed promptly without constantly applying an excess voltageto the gate of the switching element when turning the switching elementON. Therefore, this conventional example can shorten the delay time andreduce the switching losses without adding extra stress to the gate ofthe switching element. However, these conventional examples have certainshortcomings, such as, for example, not being able to reduce the amountof current consumed in the drive circuit thereof.

SUMMARY OF THE INVENTION

Embodiments of the invention address these and other shortcomings in theart. Embodiments of the invention provide a drive circuit for aninsulated gate switching element that is capable of reducing the amountof current consumed by the drive circuit driving the insulated gateswitching element.

In some embodiments, a first aspect of a drive circuit for an insulatedgate switching element according to the present invention has: aconstant current source that generates a constant current; a switchingcircuit that connects a gate of the insulated gate switching element toa power supply potential side via the constant current source whenturning the insulated gate switching element ON and connects the gate ofthe insulated gate switching element to a reference potential side via adischarge circuit when turning the insulated gate switching element OFF;a gate voltage detection circuit that detects a gate voltage of theinsulated gate switching element; and a current mode selection circuitthat switches a mode of the constant current source from a normalcurrent mode to a low current consumption mode when detecting, based onthe gate voltage detected by the gate voltage detection circuit, thatthe insulated gate switching element is turned ON.

According to this configuration, when turning the insulated gateswitching element ON, the mode of the constant current source is set atthe normal current mode and a current is injected to the gate of theinsulated gate switching element until the gate voltage reaches apredetermined voltage for turning the insulated gate switching elementON. Then, when turning the insulated gate switching element ON and thegate voltage becomes higher than the predetermined voltage, the mode ofthe constant current source is switched to the low current consumptionmode to minimize the amount of current consumed by the drive circuit.

In some embodiments, the constant current source has: a firsttransistor, a drain side of which is connected to a resistor; a secondtransistor that configures a current mirror along with the firsttransistor and generates a constant current defined by a terminalvoltage and reference voltage of the resistor; and a third transistorthat is current-mirror connected to the second transistor and has adrain connected to the gate of the insulated gate switching element.

According to this configuration, by generating the constant current atthe second transistor, a constant current corresponding to this constantcurrent is output from the third transistor to the gate of the insulatedgate switching element.

In some embodiments, the constant current source has: a firsttransistor, a drain side of which is connected to a resistor; a fourthtransistor that is connected between the first transistor and theresistor and generates a constant current defined by a terminal voltageand reference voltage of the resistor; and a third transistor thatconfigures a current mirror along with the first transistor and has adrain connected to the gate of the insulated gate switching element.

This configuration can simplify the configuration of the constantcurrent source.

In some embodiments, the current mode selection circuit sets thereference voltage at a normal voltage when the gate voltage detected bythe gate voltage detection circuit is less than a predetermined value,and sets the reference voltage at a low consumption mode voltage lowerthan the normal voltage when the gate voltage detected by the gatevoltage detection circuit is equal to or greater than the predeterminedvalue.

According to this configuration, the current mode selection circuit canset the mode of the constant current source at the normal current modewhen the gate voltage detected by the gate voltage detection circuit isless than the predetermined value, and set the mode of the constantcurrent source at the low consumption mode when the gate voltage isequal to or greater than the predetermined value.

In some embodiments, the current mode selection circuit has a partialresistor having a power supply-side resistor and ground-side resistorconnected between a positive power supply and a ground, sets aground-side partial resistance value at a normal value when the gatevoltage detected by the gate voltage detection circuit is less than apredetermined value, and sets the ground-side partial resistance valueat a low consumption mode resistance value lower than the normal valuewhen the gate voltage detected by the gate voltage detection circuit isequal to or greater than the predetermined value.

According to this configuration, a current mode selection voltage can beformed easily by changing the partial resistance value in accordancewith the gate voltage.

In some embodiments, a threshold value, based on which the gate voltagedetection circuit outputs signals of different levels depending on thegate voltage of the insulated gate switching element, is set at 7 V to14.5 V, which is the gate voltage of the insulated gate switchingelement.

According to this configuration, the insulated gate switching elementcan reliably be turned ON and the low consumption current mode can beselected by a low voltage source.

In some embodiments, the gate voltage detection circuit reduces acurrent value of the constant current source to 1/20 to ½ when the gatevoltage of the insulated gate switching element exceeds the thresholdvalue.

According to this configuration, the amount of current consumed by theentire drive circuit can significantly be minimized.

Embodiments of the invention can achieve the effect of accelerating thetime to turn the insulated gate switching element ON, with the normalcurrent mode of the constant current source, until the gate voltage ofthe insulated gate switching element reaches the predetermined voltageat which the insulated gate switching element is turned ON, and thenminimizing the amount of current consumed by the drive circuit with thelow current consumption mode of the constant current source when thegate voltage reaches the predetermined voltage at which the insulatedgate switching element is turned ON.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a first embodiment of a drivecircuit for an insulated gate switching element according to the presentinvention;

FIG. 2 is a circuit diagram showing a level shift circuit applicable tothe present invention;

FIG. 3 is a circuit showing a second embodiment of a drive circuit foran insulated gate switching element according to the present invention;and

FIG. 4 is a circuit diagram showing a conventional drive circuit for aninsulated gate switching element.

DETAILED DESCRIPTION

Embodiments of the present invention are described hereinafter withreference to the drawings.

FIG. 1 is a block diagram showing a drive circuit for an insulated gateswitching element according to the present invention.

In this diagram, reference numeral 1 represents an insulated gatebipolar transistor (referred to as “IGBT,” hereinafter), which is aninsulated gate switching element to be driven. This IGBT 1 isincorporated in, for example, an inverter circuit for converting a DC toan AC or a power conversion device such as a DC-DC converter forconverting a DC to a DC of different voltage. The insulated gateswitching element is not limited to the IGBT; thus, a power MOSFET canbe applied as the insulated gate switching element.

A drive circuit 2 for driving the IGBT 1 has a positive-side line Lpconnected to a positive power supply Vcc and a ground line Lg connectedto a ground. The drive circuit 2 also has a constant current source 3configuring a charging circuit and generating a constant current, adischarge circuit 4, a switching circuit 5, and a current mode selectioncircuit 6.

The constant current source 3 has P-channel MOSFETs (Metal OxideSemiconductor Field Effect Transistor) 11, 12, and 16 functioning asfirst, second, and third transistors that are current-mirror connectedto one another by connecting gates thereof to one another, to configurea current mirror circuit.

The P-channel MOSFET 11 has a source connected to the power supply lineLp and drain connected to the ground line Lg via the P-channel MOSFET 13and a current detecting resistor 22.

The P-channel MOSFET 12 has a source connected to the power supply lineLp and a drain connected to a drain of an N-channel MOSFET 17functioning as a fourth transistor. A source of the N-channel MOSFET 17is connected to the ground Lg.

Furthermore, the P-channel MOSFET 16 has a source connected to the powersupply line Lp and a drain connected to a gate of the IGBT 1.

The constant current source 3 has an operational amplifier 23 in which areference voltage Vref is input from the after-mentioned current modeselection circuit 6 to a non-inverting input side and a terminal voltageof the current detecting resistor 22 is input to an inverting inputterminal. An output of the operational amplifier 23 is supplied to agate of the N-channel MOSFET 17.

The P-channel MOSFETs 11, 12, and 16 have substantially the same channellength. A channel width of the P-channel MOSFET 16 is preferably atleast 10 times greater than a channel width of the P-channel MOSFET 12.Also, the resistor 22 and resistors 31 to 33 preferably have temperaturecharacteristics of 100 ppm/° C. or lower.

The discharge circuit 4 has a buffer 18 to which is input a drive signalVin configured by, for example, a pulse-width modulated (PWM) signalfrom an external controller, and an N-channel MOSFET 19 having a gateconnected to an output side of this buffer 18.

The N-channel MOSFET 19 has a source connected to the ground line Lg anda drain connected to a connection point between a source of theP-channel MOSFET 16 and the gate of the IGBT 1. Thus, a source of theN-channel MOSFET 18 is connected to an emitter of the IGBT 1 via theground line Lg.

The switching circuit 5 has a level shift circuit 20 and P-channelMOSFETS 14 and 15. The P-channel MOSFET 14 has a source connected to agate of the P-channel MOSFET 12, a body terminal connected to the powersupply line Lp, and a drain connected to a gate of the P-channel MOSFET16. The P-channel MOSFET 15 has a source connected to the power supplyline Lp and a drain connected to a connection between the drain of theP-channel MOSFET 14 and the gate of the P-channel MOSFET 16.

The level shift circuit 20 inputs the abovementioned drive signal Vin toan input terminal A and connects a non-inverting output terminal B andinverting output terminal BB to the gates of the P-channel MOSFET 14 and15 respectively.

As shown in FIG. 2, this level shift circuit 20 is configured byconnecting a series circuit consisting of a P-channel MOSFET 41,resistor 47, and N-channel MOSFET 43 between the power supply line Lpand the ground line Lg and connecting a series circuit consisting of aP-channel MOSFET 42, resistor 48, and N-channel MOSFET 44 in parallelwith the aforementioned series circuit.

A gate of the P-channel MOSFET 41 is connected to a drain of theP-channel MOSFET 42. Similarly, a gate of the P-channel MOSFET 42 isconnected to a drain of the P-channel MOSFET 41. Furthermore, zenerdiodes 45 and 46 are connected in parallel with the P-channel MOSFETS 41and 42 respectively.

The inverting output terminal BB is derived from a connection pointbetween an anode of the zener diode 45 and the drain of the P-channelMOSFET 41, and a normal rotation output terminal B is derived from aconnection point between an anode of the zener diode 46 and the drain ofthe P-channel MOSFET 42. Moreover, gates of the N-channel MOSFETs 43 and44 are connected to each other by a logical inversion circuit 49. Aconnection point between the gate of the N-channel MOSFET 43 and thelogical inversion circuit 49 is the input terminal A.

When the level of the drive signal that is input to the input terminal Ais high, the level shift circuit 20 turns the N-channel MOSFET 43 ON andthe N-channel MOSFET 44 OFF. Consequently, the P-channel MOSFET 42 isturned ON and the P-channel MOSFET 41 is turned OFF. Therefore, ahigh-level output signal is output from the normal rotation outputterminal B, and a low-level output signal is output from the invertingoutput terminal BB.

On the other hand, when the level of the drive signal Vin is low, thelevel shift circuit 20 turns the N-channel MOSFET 43 OFF and theN-channel MOSFET 44 ON. Consequently, the P-channel MOSFET 41 is turnedON and the P-channel MOSFET 42 is turned OFF. Therefore, the level ofthe output signal from the normal rotation output terminal B becomeslow, and the level of the output signal that is output to the invertingoutput terminal BB becomes high.

The current mode selection circuit 6 has the power supply-side resistor31 that is connected serially to the power supply line Lp and the groundline Lg, and the ground-side resistors 32 and 33. The current modeselection circuit 6 connects the bypass N-channel MOSFET 34 in parallelwith the ground-side resistor 33, and connects the output side of thebuffer 35 to a gate of this N-channel MOSFET 34.

On the other hand, a gate voltage detection circuit 7 is connected tothe connection point between the drain of the P-channel MOSFET 16 andthe gate of the IGBT 1. The gate voltage detection circuit 7 isconfigured to detect that a gate voltage Vg becomes equal to or greaterthan a predetermined voltage Vs (e.g., 13 V) that is sufficiently higherthan a threshold voltage of the IGBT 1. When Vg is less than Vs, thegate voltage detection circuit 7 outputs a gate voltage detection signalVdg of a low level to the buffer 35 of the current mode selectioncircuit 6. When Vg is equal to or greater than Vs, the gate voltagedetection circuit 7 outputs the gate voltage detection signal Vdg of ahigh level to the buffer 35 of the current mode selection circuit 6. Itis preferred that the predetermined voltage Vs of the gate voltagedetection circuit 7 be set at a gate voltage of 7 V to 14.5 V at whichthe IGBT 1 can surely be turned ON.

In the current mode selection circuit 6, therefore, when the gatevoltage Vg of the IGBT 1 detected by the gate voltage detection circuit7 is less than the predetermined threshold Vs (Vg<Vs), the level of thegate voltage detection signal Vdg becomes low, thereby maintaining theOFF state of the N-channel MOSFET 34. As a result, the ground-sideresistor 33 remains connected to the ground-side resistor 32, and areference voltage Vref1 that is output from the connection point betweenthe power supply-side resistor 31 and the ground-side resistor 32 isexpressed by the following formula (1) where R1 represents a resistancevalue of the power supply-side resistor 31 and R2 and R3 representresistance values of the ground-side resistors 32 and 33 respectively.

Vref1={(R2+R3)/(R1+R2+R3)}Vcc   (1)

At the gate voltage Vg that is equal to or greater than thepredetermined voltage Vs enough to turn the IGBT 1 ON, this gate voltageVg is detected by the gate voltage detection circuit 7. Consequently,the level of the gate voltage detection signal Vdg becomes high, andthereby the N-channel MOSFET 34 is turned ON. As a result, theground-side resistor R3 is bypassed by the N-channel MOSFET 34,connecting the ground-side resistor 32 to the ground line Lg via theN-channel MOSFET 34. Therefore, a reference voltage Vref2 that is outputfrom the connection point between the power supply-side resistor 31 andthe ground-side resistor 32 is expressed by the following formula (2).

Vref2={(R2)/(R1+R2)}Vcc   (2)

In this case, when the level of the gate voltage detection signal Vdg ofthe gate voltage detection circuit 7 is low at the time of: the powersupply voltage Vcc=15 V; the resistance value R1 of the resistor 31=50kΩ; the resistance value R2 of the resistor 32=2 kΩ; the resistancevalue R3 of the resistor 33=10 kΩ; and the resistance value R4 of theresistor 22=2 kΩ, the reference voltage Vref1 becomes 2.90 V, and amirror current 11 flowing through the N-channel MOSFET 17 becomes 1.45mA.

When, on the other hand, the level of the gate voltage detection signalVdg of the gate voltage detection circuit 7 is high, the referencevoltage Vref2 becomes 0.58 V, and the mirror current 11 flowing throughthe N-channel MOSFET 17 becomes 0.29 mA. In other words, the mirrorcurrent becomes approximately ⅕ of the mirror current obtained when thelevel of gate voltage detection signal Vdg is low.

Next, operations according to the first embodiment are described.

When a high-level signal is input from the external controller to thedrive signal Vin as a signal for turning the IGBT OFF, the level of theoutput signal from the normal rotation output terminal B becomes highand the level of the output signal from the inverting output terminal BBbecomes low in the level shift circuit 20. When the pulse-widthmodulated signal is not input as the drive signal, high-level signalsare constantly input to the drive signal Vin in order to keep the IGBTOFF.

As a result, the P-channel MOSFET 14 is turned OFF, the P-channel MOSFET15 is turned ON, and the P-channel MOSFET 16 is turned OFF. On the otherhand, in the discharge circuit, the N-channel MOSFET 19 is turned ON.For this reason, the gate of the IGBT 1 is not injected with current,i.e., not charged, through the constant current source 3. By connectingthe gate of the IGBT 1 to the ground line Lg via the N-channel MOSFET 19of the discharge circuit 4, a discharge state of the IGBT 1 ismaintained. Accordingly, the IGBT 1 is turned OFF or kept OFF.

When the level of the drive signal Vin becomes low in the OFF state ofthe IGBT 1, the level of the output signal from the normal rotationoutput terminal B of the level shift circuit 20 becomes low, and thelevel of the output signal from the inverting output terminal BB becomeshigh. As a result, the P-channel MOSFET 14 is turned ON, and theP-channel MOSFET 15 is turned OFF. This, consequently, configures thecurrent mirror circuit in the constant current source 3, and a mirrorcurrent I11 is injected into the gate of the IGBT 1 through theP-channel MOSFET 16, charging the IGBT 1.

At this moment, because the gate voltage Vg of the IGBT 1 is lower thanthe predetermined voltage Vs, the gate voltage detection signal Vdg thatis output from the gate voltage detection circuit 7 keeps its level low.As a result, the N-channel MOSFET 34 of the current mode selectioncircuit 6 is kept OFF, and the reference voltage Vref1 of a relativelyhigh level (=2.90 V) is supplied to a non-inverting input terminal ofthe operational amplifier 23. Because the output signal of theoperational amplifier 23 produces a relatively high voltage, theN-channel MOSFET 17 is turned ON, and the mirror current I1 of arelatively high level flows thereto. Therefore, the mode of the constantcurrent source 3 becomes the normal current mode where a current kI1that is k times the mirror current I1 is injected from the P-channelMOSFET 16 to the gate of the IGBT 1. As a result, the IGBT 1 can beturned ON by means of the constant current source 3.

At this moment, the gate voltage Vg increases due to the injection ofthe current kI1 from the P-channel MOSFET 16 to a gate capacitance ofthe IGBT 1. When the gate voltage Vg reaches the predetermined voltageVs that is sufficiently higher than the threshold voltage of the IGBT 1,the gate voltage detection signal Vdg of a high level is output from thegate voltage detection circuit 7. Because this gate voltage detectionsignal Vdg is supplied to the buffer 35 of the current mode selectioncircuit 6, the N-channel MOSFET 34 is turned ON. Consequently, theground-side resistor 33 is bypassed, connecting the ground-side resistor32 directly to the ground line Lg.

Therefore, the reference voltage decreases from Vref1 (=2.90 V) to Vref2(=0.58V), and the output voltage of the operational amplifier 23decreases as well. As a result, the mirror current I1 flowing throughthe N-channel MOSFET 17 is reduced to approximately ⅕, as describedabove. For this reason, the mirror current kI1, k times the mirrorcurrent I1, which is supplied to the IGBT 1 through the P-channel MOSFET16, also drops. However, the gate capacitance of the IGBT 1 keeps itscharged state and is therefore kept ON.

Because a rise time of the gate voltage is extremely shorter than aswitching cycle of the drive signal Vin, a time period in which the gatevoltage is less than the predetermined voltage Vs when the IGBT 1 isturned ON is extremely short. Therefore, the average value of the mirrorcurrent I1 becomes approximately equivalent to 0.29 mA obtained when thelevel of the gate voltage detection signal Vdg of the gate voltagedetection circuit 7 is high. As a result, the mode of the constantcurrent source can be set at the low power consumption mode.

Incidentally, in the case of the conventional example that is notprovided with the gate voltage detection circuit 7 and the current modeselection circuit 6, the mirror current I1 in an amount of 1.45 mAcontinuously flows during the period in which the IGBT 1 is ON, causingthe drive circuit to keep the state of high current consumption.

In the present embodiment, however, the consumption of current can bereduced by 1.45 mA−0.29 mA=1.16 mA during the period in which the IGBT 1is ON. Because the period in which the IGBT 1 is ON becomesapproximately half a calmative time of the drive signal Vin, the amountof current consumed can be reduced by approximately 1.16 mA/2=0.58 mA inthe normal switching state.

However, when the level of the drive signal Vin becomes high, the levelof the output signal that is output from the normal rotation outputterminal B of the level shift circuit 20 becomes high, whereas the levelof the output signal that is output from the inverting output terminalBB becomes low. As a result, the P-channel MOSFET 14 is turned OFF, andthe P-channel MOSFET 15 is turned ON. Furthermore, the P-channel MOSFET16 is turned OFF, stopping the injection of the current kI1 into theIGBT 1.

At the same time, because the drive signal Vin of a high level is inputto the buffer 18 of the discharge circuit 4, the N-channel MOSFET 19 ofthis discharge circuit 4 is turned ON. As a result, a gate charge of theIGBT 1 is drawn to the ground line Lg through the P-channel MOSFET 19,whereby the IGBT 1 enters the discharge state, reducing the gate voltageand turning the IGBT 1 OFF.

According to the first embodiment described above, when turning the IGBT1 ON and when the gate voltage Vg is less than the predetermined voltageVs sufficiently higher than the threshold voltage for turning the IGBT 1ON, the reference voltage Vref1 of a high level is supplied to theoperational amplifier 23 by means of the current mode selection circuit6, and the mirror current I1 of the constant current source 3 is set ata normal current value to set the mode of the constant current source 3at the normal current mode. Subsequently, when the gate voltage Vgreaches the predetermined voltage Vs, the reference voltage Vref2 of alow level is supplied to the operation amplifier 23 by means of thecurrent mode selection circuit 6, and the mirror current I1 of theconstant current source 3 is lowered to approximately ⅕ of the normalcurrent value, to set the mode of the constant current source 3 at thelow current consumption mode. As a result, the amount of currentconsumed by the drive circuit can be reduced, while accelerating thetime to turn the IGBT 1 ON.

Furthermore, when the gate potential of the IGBT 1 changes from theground potential to the power supply potential Vcc, the voltage that issubstantially the same as a gate potential of the IGBT 21 can be appliedto the drain of the P-channel MOSFET 11 by placing the P-channel MOSFET13 between the P-channel MOSFET 11 and the resistor 22 of the constantcurrent source 3. Consequently, the current balance between the currentmirrors of the P-channel MOSFETs 11 and 16 can be maintained. For thisreason, the level of current flowing through the P-channel MOSFET 16 canbe kept constant, regardless of the gate potential of the IGBT 1 (an OUTterminal voltage of the drive circuit).

Moreover, a stable constant current can be generated by providing theconstant current source 3 with the P-channel MOSFET 11 functioning asthe first transistor for detecting a constant current and the P-channelMOSFET 12 functioning as the second transistor for controlling theconstant current.

In addition, changes can be made easily to the reference voltage Vrefbased on the gate voltage detection signal Vdg, by configuring thecurrent mode selection circuit 6 with the partial resistors 31 to 33 andthe switching element 34.

Next, a second embodiment of the present invention is described withreference to FIG. 3.

The second embodiment is obtained by simplifying the configuration ofthe constant current source by omitting the P-channel MOSFETs 12 and 13functioning as the second and fourth transistors in the first embodimentdescribed above.

In other words, in the second embodiment, the P-channel MOSFETs 12 and13 functioning as the second and fourth transistors are omitted from theconfiguration shown in FIG. 1 of the first embodiment. Also, theN-channel MOSFET 17, the gate of which receives input of the outputsignal from the operational amplifier 23, is connected between theP-channel MOSFET 11 functioning as the first transistor and the resistor22. The rest of the configuration is the same as the one shown in FIG.1; thus, the same reference numerals are applied to the sectionscorresponding to the ones shown in FIG. 1, and the detailed descriptionsthereof are omitted accordingly.

In the current mode selection circuit 6, when the level of the gatevoltage detection signal Vdg of the gate voltage detection circuit 7 islow under the conditions that the power supply voltage Vcc=15 V, theresistance value R1 of the resistor 31=20 kΩ, the resistance value R2 ofthe resistor 32=1 kΩ, the resistance value R3 of the resistor 33=5 kΩ,and the resistance value R4 of the resistor 22=3 kΩ, the referencevoltage Vref1 becomes 3.46 V based on the formula (1) providedpreviously, and the mirror current I1 flowing through the N-channelMOSFET 17 becomes 1.15 mA.

When, on the other hand, the level of the gate voltage detection signalVdg of the gate voltage detection circuit 7 is high, the referencevoltage Vref2 becomes 0.71 V based on the formula (2) providedpreviously, and the mirror current 11 flowing through the N-channelMOSFET 17 becomes 0.24 mA, which is approximately ⅕ of the mirrorcurrent obtained when the level of the gate voltage detection signal Vdgis low.

According to the second embodiment, the drive signal Vin configured by apulse-width modulated (PWM) signal is kept at a high level when, forexample, the drive signal Vin is not input from the external controller.As a result, the level of the output signal from the normal rotationoutput terminal B of the level shift circuit 20 becomes high, whereasthe level of the output signal from the inverting output terminal BBbecomes low.

Consequently, the P-channel MOSFET 14 is turned OFF, and the P-channelMOSFET 15 is turned ON. The P-channel MOSFET 16 is turned OFF as well.In the discharge circuit 4, on the other hand, the N-channel MOSFET 19is turned ON.

Therefore, current injection (charging) into the gate of the IGBT 1through the constant current source 3 is not carried out, and connectingthe gate of the IGBT 1 to the ground line Lg through the N-channelMOSFET 19 of the discharge circuit 4 keeps the discharge state of theIGBT 1, keeping the OFF state of the IGBT 1.

When the level of the drive signal Vin becomes low in this OFF state ofthe IGBT 1, the level of the output signal from the normal rotationoutput terminal B of the level shift circuit 20 become low, whereas thelevel of the output signal from the inverting output terminal BB becomeshigh. As a result, the P-channel MOSFET 14 is turned ON, and theP-channel MOSFET 15 is turned OFF. Accordingly, a current mirror circuitis configured in the constant current source 3, wherein the mirrorcurrent I11 is supplied to the gate of the IGBT 1 through the P-channelMOSFET 16, starting current injection, or charging, on the IGBT 1.

At this moment, because the gate voltage Vg of the IGBT 1 is lower thanthe predetermined threshold voltage Vs, the level of the gate voltagedetection signal Vdg that is output from the gate voltage detectioncircuit 7 is kept low. Therefore, the N-channel MOSFET 34 of the currentmode selection circuit 6 is kept OFF, and the reference voltage Vref1 ofa relatively high level (=3.46 V) is supplied to the non-inverting inputterminal of the operational amplifier 23. As a result, the output signalof the operational amplifier 23 becomes a relatively high voltage.Consequently, the N-channel MOSFET 17 is turned ON, allowing the mirrorcurrent I1 of a relatively high level (=1.15 mA) to flow. Accordingly,the current kI1, which is k times the mirror current I1, is injectedfrom the P-channel MOSFET 16 into the gate of the IGBT 1. As a result,the IGBT 1 is turned ON via the constant current source 3.

Here, when the gate voltage Vg rises and reaches the predeterminedvoltage Vs sufficiently higher than the threshold voltage of the IGBT 1as a result of the injection of the current kI1 of the P-channel MOSFET16 into the gate capacitance of the IGBT 1, the gate voltage detectionsignal Vdg of a high level is output from the gate voltage detectioncircuit 7. This gate voltage detection signal Vdg is supplied to thebuffer 35 of the current mode selection circuit 6, turning the N-channelMOSFET 34 ON. As a result, the ground-side resistor 33 is bypassed, andthe ground-side resistor 32 is connected directly to the ground line Lg.

In response thereto, the reference voltage decreases from Vref1 (=3.46V) to Vref2 (=0.71 V), and the output voltage of the operationalamplifier 23 decreases as well. Consequently, the mirror current 11flowing through the N-channel MOSFET 17 is reduced to approximately ⅕,as described above. For this reason, the mirror current kI1, k times themirror current I1, which is supplied to the IGBT 1 through the P-channelMOSFET 16, also drops. However, the gate capacitance of the IGBT 1 keepsits charged state and is therefore kept ON.

Because the rise time of the gate voltage is extremely shorter than theswitching cycle of the drive signal Vin, a time period in which the gatevoltage is less than the predetermined voltage Vs when the IGBT 1 is ONis extremely short. Therefore, the average value of the mirror currentI1 becomes approximately equivalent to 0.24 mA obtained when the levelof the gate voltage detection signal Vdg of the gate voltage detectioncircuit 7 is high. As a result, the mode of the constant current sourcecan be set at the low power consumption mode.

Incidentally, in the case of the conventional example that is notprovided with the gate voltage detection circuit 7 and the current modeselection circuit 6, the mirror current I1 in an amount of 1.15 mAcontinuously flows during the period in which the IGBT 1 is ON, causingthe drive circuit to keep the state of high current consumption.

In the present embodiment, however, the consumption of current can bereduced by 1.15 mA−0.24 mA=0.91 mA during the period in which the IGBT 1is ON. Because the period in which the IGBT 1 is ON becomesapproximately half the calmative time of the drive signal Vin, theamount of current consumed can be reduced by approximately 0.91mA/2=0.455 mA in the normal switching state.

However, when the level of the drive signal Vin becomes high, the levelof the output signal that is output from the normal rotation outputterminal B of the level shift circuit 20 becomes high, whereas the levelof the output signal that is output from the inverting output terminalBB becomes low. As a result, the P-channel MOSFET 14 is turned OFF, andthe P-channel MOSFET 15 is turned ON. Furthermore, the P-channel MOSFET16 is turned OFF, stopping the injection of the current kI1 into theIGBT 1.

At the same time, because the drive signal Vin of a high level is inputto the buffer 18 of the discharge circuit 4, the N-channel MOSFET 19 ofthis discharge circuit 4 is turned ON. As a result, the gate charge ofthe IGBT 1 is drawn to the ground line Lg through the P-channel MOSFET19, whereby the IGBT 1 enters the discharge state, reducing the gatevoltage and turning the IGBT 1 OFF.

According to the second embodiment described above, when turning theIGBT 1 ON and when the gate voltage Vg is less than the predeterminedvoltage Vs sufficiently higher than the threshold voltage for turningthe IGBT 1 ON, the reference voltage Vref1 of a high level is suppliedto the operational amplifier 23 by means of the current mode selectioncircuit 6, and the mirror current I1 of the constant current source 3 isset at the normal current value, to set the mode of the constant currentsource 3 at the normal current mode. Subsequently, when the gate voltageVg reaches the predetermined voltage Vs, the reference voltage Vref2 ofa low level is supplied to the operation amplifier 23 by means of thecurrent mode selection circuit 6, and the mirror current I1 of theconstant current source 3 is lowered to approximately ⅕ of the normalcurrent value to set the mode of the constant current source 3 at thelow current consumption mode. As a result, the amount of currentconsumed by the entire drive circuit can be reduced, while acceleratingthe time to turn the IGBT 1 ON.

In the first and second embodiments, the IGBT 1 is applied as aninsulated gate switching element; however, the IGBT 1 is not limitedthereto, and a different type of insulated gate switching element suchas a MOSFET may be applied.

In the first and second embodiments, the current value obtained in thelow consumption mode is lower than the current value obtained in thenormal current mode by approximately ⅕; however, the amount of reductionis not limited thereto. Thus, when the gate voltage Vg of the insulatedgate switching element is equal to or greater than the predeterminedvoltage Vs, the current value of the constant current source 3 may bereduce to a value that is 1/20 to ½ of the current value obtained in thenormal current mode. When the current value obtained in the low currentconsumption mode of the constant current source 3 is less than 1/20 ofthe current value obtained in the normal current mode, the value of thecurrent applied to the gate of the insulated gate switching element istoo low to keep the insulated gate switching element ON. When thecurrent value is less than ½ the current value obtained in the normalcurrent mode, the effect of reducing the current consumption becomeslow.

In the first and second embodiments, the MOSFETs are applied as activeelements of the constant current source 3, the discharge circuit 4, theswitching circuit 5, and the current mode selection circuit 6; however,the active elements are not limited to the MOSFETs. Any active elementssuch as FET and bipolar transistors can be applied.

Embodiments of the invention can provide a drive circuit for aninsulated gate switching element, which is capable of accelerating thetime to turn ON the insulated gate switching element, with a normalcurrent mode of a constant current source, until a gate voltage of theinsulated gate switching element reaches a predetermined voltage forturning the insulated gate switching element ON, and of minimizing theamount of current consumed by the drive circuit, with a low currentconsumption mode of the constant current source, once the gate voltagereaches the predetermined voltage for turning the insulated gateswitching element ON.

Examples of specific embodiments are illustrated in the accompanyingdrawings. While the invention is described in conjunction with thesespecific embodiments, it will be understood that it is not intended tolimit the invention to the described embodiments. On the contrary, it isintended to cover alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims. In the above description, specific details are setforth in order to provide a thorough understanding of embodiments of theinvention. Embodiments of the invention may be practiced without some orall of these specific details. Further, portions of differentembodiments and/or drawings can be combined, as would be understood byone of skill in the art.

What is claimed is:
 1. A drive circuit for an insulated gate switchingelement, comprising: a constant current source that generates a constantcurrent; a switching circuit that connects a gate of the insulated gateswitching element to a power supply potential side via the constantcurrent source when turning the insulated gate switching element ON andconnects the gate of the insulated gate switching element to a referencepotential side via a discharge circuit when turning the insulated gateswitching element OFF; a gate voltage detection circuit that detects agate voltage of the insulated gate switching element; and a current modeselection circuit that switches a mode of the constant current sourcefrom a normal current mode to a low current consumption mode whendetecting, based on the gate voltage detected by the gate voltagedetection circuit, that the insulated gate switching element is turnedON.
 2. The drive circuit for an insulated gate switching elementaccording to claim 1, wherein the constant current source has: a firsttransistor, a drain side of which is connected to a resistor; a secondtransistor that configures a current mirror along with the firsttransistor and generates a constant current defined by a terminalvoltage and reference voltage of the resistor; and a third transistorthat is current-mirror connected to the second transistor and has adrain connected to the gate of the insulated gate switching element. 3.The drive circuit for an insulated gate switching element according toclaim 1, wherein the constant current source has: a first transistor, adrain side of which is connected to a resistor; a fourth transistor thatis connected between the first transistor and the resistor and generatesa constant current defined by a terminal voltage and reference voltageof the resistor; and a third transistor that configures a current mirroralong with the first transistor and has a drain connected to the gate ofthe insulated gate switching element.
 4. The drive circuit for aninsulated gate switching element according to claim 1, wherein thecurrent mode selection circuit sets the reference voltage at a normalvoltage when the gate voltage detected by the gate voltage detectioncircuit is less than a predetermined value, and sets the referencevoltage at a low consumption mode voltage lower than the normal voltagewhen the gate voltage detected by the gate voltage detection circuit isequal to or greater than the predetermined value.
 5. The drive circuitfor an insulated gate switching element according to claim 1, whereinthe current mode selection circuit has a partial resistor having a powersupply-side resistor and ground-side resistor connected between apositive power supply and a ground, sets a ground-side partialresistance value at a normal value when the gate voltage detected by thegate voltage detection circuit is less than a predetermined value, andsets the ground-side partial resistance value at a low consumption moderesistance value lower than the normal value when the gate voltagedetected by the gate voltage detection circuit is equal to or greaterthan the predetermined value.
 6. The drive circuit for an insulated gateswitching element according to claim 1, wherein a threshold value, basedon which the gate voltage detection circuit outputs signals of differentlevels depending on the gate voltage of the insulated gate switchingelement, is set at 7 V to 14.5 V, which is the gate voltage of theinsulated gate switching element.
 7. The drive circuit for an insulatedgate switching element according to claim 6, wherein the gate voltagedetection circuit reduces a current value of the constant current sourceto 1/20 to ½ when the gate voltage of the insulated gate switchingelement exceeds the threshold value.